Stopping starlight may bring other Earths into focus

SUN FLOWER A starshade accompanying a proposed planet-hunting telescope, shown in an illustration, would block starlight and make it easier for the telescope to glimpse any planets orbiting distant stars.

BOSTON — When Voyager 1 pivoted back toward Earth from beyond Neptune in 1990, it snapped one of the most famous space pictures: the pale blue dot, with Earth appearing as a lonely speck of light. Astronomers are now designing a new generation of telescopes with hopes of taking a photo of another pale blue dot, this one orbiting a distant star. The proposals offer two contrasting ways of blocking out a distant star’s light, one with a giant shade traveling through space near the telescope and the other with shape-changing mirrors within the telescope.

NASA’s crippled Kepler space telescope has already shown that small rocky planets are common (SN: 4/5/14, p. 15). But Kepler’s data can’t distinguish Earthlike planets from dead ones more like Venus, Mars or something unimagined. On June 4 at the American Astronomical Society meeting, astronomers presented ideas for spacecraft that could both get pictures of rocky worlds and also measure the chemical makeup of their atmospheres.

Snapping a picture of an exoplanet is fiendishly difficult. As seen from Earth, a planet hugs its parent star. Adding to the challenge, stars are roughly 1 billion times brighter in visible light than any planets. Anyone who has ever tried to see something while looking toward the sun knows what to do: Block the sun with your hand. Turns out that strategy should work just as well for seeing exoplanets.

Sara Seager, an astrophysicist at MIT, presented a telescope design called Exo-S that uses the idea by pairing a simple telescope in space with a starshade. The shade is a 34-meter-wide flower-shaped disk floating tens of thousands of kilometers away that prevents starlight from entering the telescope. The starshade’s unusual construction — 28 petals, each 7 meters long, that unfurl and snap into place — minimizes how much light leaks into the telescope.

The design’s big advantage, said Seager, is that it doesn’t require a big, complex telescope. Without the starshade, the telescope would need a large mirror to catch dim light from a planet and see objects huddled up close to a star. With a starshade, she said, the telescope could, in principle, be just 1 centimeter in diameter. In practice, Exo-S would need a telescope just over a meter across, which is still less than half the diameter of the Hubble Space Telescope.

A very different proposal for blocking starlight called Exo-C calls for putting a small light-blocking disk, called a coronagraph, inside the telescope. The telescope itself would require a more complicated design than that of Exo-S. To precisely steer the scattered starlight, Exo-C would rely on adaptive optics, mirrors that change shape as needed.

“The amazing thing is that the deformable mirror technology exists to do this,” says Karl Stapelfeldt, an astrophysicist at NASA Goddard Space Flight Center, in Greenbelt, Md. The ability to distinguish a point of light from another one that is a billion times brighter has already been demonstrated in the laboratory, he said.

But a simple picture of a dot doesn’t help astronomers distinguish living worlds from dead ones. To search for a true Earth twin, both designs include a spectrometer, a device that splits light reflecting off the exoplanet into different colors. The spectrometer will record the chemical mixtures in exoplanet atmospheres. If alien life is anything like life on Earth, it will leave chemical traces — like oxygen and methane — in the atmosphere.

“Both are really cool concepts,” says Nikole Lewis, another MIT astrophysicist, adding that like Exo-S, Exo-C has pros and cons. The coronagraph is similar to what some ground-based planet-hunting telescopes use already. And Exo-C could respond to ground-based discoveries quickly by refocusing on a potential planet-holding star.

Exo-S, by contrast, would require days or weeks to move its view from one star to another because of the challenges involved with moving the telescope and the starshade in tandem. Starshades are also mostly untested. However, compared with the coronagraph design, Exo-S may be able to see fainter planets that are closer to their stars. While the starshade technology isn’t as mature, says Lewis, “it has a better chance of seeing a pale blue dot.”

The designs were developed in response to a 2013 request from NASA for innovative exoplanet missions that could be completed for less than $1 billion. These would complement a number of early-stage exoplanet missions that NASA already supports that target different types of stars and planets. Over the next decade, the TESS, JWST and WFIRST-AFTA telescopes will include exoplanets in their missions, though TESS — the Transiting Exoplanet Survey Satellite — is the only one designed specifically for planet hunting.

Astronomers are still fleshing out the details for these competing concepts; final reports are due to NASA in January 2015. Whichever concept is selected, NASA hopes to start design work in 2017 with a launch date around 2024. If either proposal is successful, humankind may get its first direct glimpse of an Earth twin before the end of the next decade.

A starshade, shown in an animation and then in a model demonstration, would shield a proposed planet-hunting telescope from starlight, making it easier for the telescope to glimpse any planets orbiting distant stars. Credit: NASA Jet Propulsion Laboratory

Editor's Note: This story was updated on June 10, 2014, to correct the planned distance between the starshade and the telescope in the Exo-S design. It was further updated on June 12, 2014, to provide the correct distance to, and name of, the spacecraft that took the Pale Blue Dot photo.